Method of producing cold rolled steel strip and production system for cold rolled steel strip

ABSTRACT

A method of producing a cold rolled steel strip includes: subjecting a steel strip that has been cold rolled and subsequently continuously annealed to pickling by continuously feeding the steel strip into a mixed acid solution containing a first acid that is oxidizing and a second acid that is non-oxidizing to immerse the steel strip; and subsequently subjecting the steel strip to repickling by continuously feeding the steel strip into an acid solution containing a third acid that is non-oxidizing to immerse the steel strip. The concentration of the first acid in the mixed acid solution is lowered and the concentration of the second acid in the mixed acid solution is raised as the iron ion concentration in the mixed acid solution rises.

TECHNICAL FIELD

This disclosure relates to a method of producing a cold rolled steelstrip and a production system for a cold rolled steel strip.

BACKGROUND

In recent years, there has been strong demand for improved automobilefuel efficiency from a viewpoint of global environmental protection.There has also been strong demand for strengthening of automotive bodiesfrom a viewpoint of ensuring passenger safety upon collision. To respondto these demands, the simultaneous achievement of both weight-reductionand strengthening of automotive bodies is being actively promotedthrough strengthening and sheet metal thinning (weight-reduction) ofcold rolled steel sheets used as a material for automotive components.However, since many automotive components are produced through formingof a cold rolled steel sheet, the cold rolled steel sheet serving as amaterial for these components is required to have excellent formabilityin addition to high strength.

There are various methods for increasing the strength of a cold rolledsteel sheet such as solid solution strengthening through Si addition,which is an effective means for strengthening without significant lossof formability. However, the addition of a large amount of Si to a coldrolled steel sheet, and particularly the addition of 0.5 mass % or moreof Si, is known to result in the formation of a large amount ofSi-containing oxides such as SiO₂ and Si—Mn-based composite oxides atthe steel sheet surface during slab reheating, during hot rolling, orduring annealing after cold rolling. These Si-containing oxidessignificantly reduce chemical convertibility such that high-strengthcold rolled steel sheets containing a large amount of Si have poorchemical convertibility. Moreover, high-strength cold rolled steelsheets containing a large amount of Si suffer from a problem of havingpoor post-coating corrosion resistance and being more susceptible tocoating peeling than normal cold rolled steel sheets when exposed to aharsh corrosive environment after electrodeposition coating, such as ina warm salt water immersion test or a wet-dry combined cyclic corrosiontest. Consequently, it is difficult to use high-strength cold rolledsteel sheets containing a large amount of Si in body applications forwhich coating is essential.

Patent literature (PTL) 1 and 2 provide techniques for solving thisproblem. PTL 1 and 2 each describe a method of producing a cold rolledsteel sheet including subjecting a steel sheet that has been cold rolledand subsequently continuously annealed to pickling by continuouslyfeeding the steel sheet into a mixed acid (nitric acid and hydrochloricacid, nitric acid and hydrofluoric acid, or the like) to immerse thesteel sheet, and subsequently subjecting the steel sheet to repicklingby continuously feeding the steel sheet into a non-oxidizing acid(hydrochloric acid, sulfuric acid, or the like) to immerse the steelsheet. The described method removes Si-containing oxides at the steelsheet surface through the pickling and removes iron-based oxides thatare produced in the pickling through the repickling, and thereby enablesproduction of a cold rolled steel sheet having excellent chemicalconvertibility and post-coating corrosion resistance in harsh corrosiveenvironments.

CITATION LIST Patent Literature

PTL 1: JP 2012-132092 A

PTL 2: JP 2012-188693 A

SUMMARY Technical Problem

However, our studies have demonstrated that when a cold rolled steelstrip is continuously passed along a production system capable ofimplementing two-stage pickling such as described above, and the coldrolled steel strip is continuously subjected to this two-stage picking,as time passes, the surface appearance quality of the cold rolled steelstrip produced at that time tends to become poorer. Specifically, werealized that as time passes, the surface of the cold rolled steel stripstraight after the pickling in the first stage is discolored to areddish-brown color due to adhered matter, and this discoloration is notremoved by the repickling in the second stage. Among cold rolled steelstrips having poor surface appearance quality as described above, wefound that there were cold rolled steel strips that also had poorchemical convertibility and post-coating corrosion resistance in harshcorrosive environments.

In view of the problems set forth above, it would be beneficial toprovide a method of producing a cold rolled steel strip and a productionsystem for a cold rolled steel strip that enable continuous productionwith long-term stability of a cold rolled steel strip having excellentchemical convertibility, post-coating corrosion resistance in harshcorrosive environments, and surface appearance quality.

Solution to Problem

Through diligent studies, we discovered that there is a correlationbetween the surface appearance quality of a cold rolled steel strip andthe iron ion concentration (hereinafter, also referred to simply as the“Fe concentration”) in a mixed acid used when the cold rolled steelstrip is subjected to pickling in the first stage. Specifically, wediscovered that the surface of a cold rolled steel strip pickled withthis mixed acid had a higher tendency to be discolored to areddish-brown color when the Fe concentration in the mixed acid washigher.

We studied the cause of this and found that the picking rate increasesas Fe gradually elutes from a cold rolled steel sheet during thepickling and the Fe concentration in the mixed acid rises. As a result,the reaction heat that is generated exceeds the cooling capability ofequipment for circulating the mixed acid and the temperature of themixed acid rises. Moreover, we realized that when the cold rolled steelstrip exits a pickling tank into the atmosphere, drying is promoted anddiscoloration occurs due to drying proceeding in a state in which mixedacid solution remains on the cold rolled steel strip. Therefore,although it is a prerequisite that a certain degree of pickling weightloss is ensured from a viewpoint of securing good chemicalconvertibility and post-coating corrosion resistance, it is alsonecessary to appropriately control the pickling rate (i.e., the mixedacid temperature) in accordance with the Fe concentration in the mixedacid in order that surface appearance quality does not deteriorate.

This disclosure is based on the findings described above and has thefollowing primary features.

(1) A method of producing a cold rolled steel strip comprising:

subjecting a steel strip that has been cold rolled and subsequentlycontinuously annealed to pickling by continuously feeding the steelstrip into a mixed acid solution containing a first acid that isoxidizing and a second acid that is non-oxidizing to immerse the steelstrip; and

subsequently subjecting the steel strip to repickling by continuouslyfeeding the steel strip into an acid solution containing a third acidthat is non-oxidizing to immerse the steel strip, wherein

concentration of the first acid in the mixed acid solution is loweredand concentration of the second acid in the mixed acid solution israised as iron ion concentration in the mixed acid solution rises.

(2) The method of producing a cold rolled steel strip according to theforegoing (1), wherein

the first acid is nitric acid.

(3) The method of producing a cold rolled steel strip according to theforegoing (1) or (2), wherein

at least one of the second acid and the third acid is at least oneselected from the group consisting of hydrochloric acid, sulfuric acid,phosphoric acid, pyrophosphoric acid, formic acid, acetic acid, citricacid, hydrofluoric acid, and oxalic acid.

(4) The method of producing a cold rolled steel strip according to theforegoing (1), wherein

the first acid is nitric acid, and the second acid and the third acidare hydrochloric acid.

(5) The method of producing a cold rolled steel strip according to theforegoing (4), wherein

concentration of the nitric acid in the mixed acid solution is setwithin a range of higher than 110 g/L and not higher than 188 g/L, andconcentration of the hydrochloric acid in the mixed acid solution is setwithin a range of higher than 4.5 g/L and not higher than 12.5 g/L.

(6) The method of producing a cold rolled steel strip according to anyone of the foregoing (1) to (5), further comprising

immersing the steel strip in water after the pickling and before therepickling.

(7) The method of producing a cold rolled steel strip according to anyone of the foregoing (1) to (6), wherein t

he picking and the repickling have a total pickling weight loss of 8g/m² or more.

(8) The method of producing a cold rolled steel strip according to anyone of the foregoing (1) to (7), wherein the steel strip contains 0.5mass % to 3.0 mass % of Si.

(9) A production system for a cold rolled steel strip comprising:

a first stock solution tank holding a stock solution of a first acidthat is oxidizing, a second stock solution tank holding a stock solutionof a second acid that is non-oxidizing, and a third stock solution tankholding a stock solution of a third acid that is non-oxidizing;

a first pipe extending from the first stock solution tank, a second pipeextending from the second stock solution tank, and a third pipeextending from the third stock solution tank;

a mixed acid solution circulation tank to which the first pipe and thesecond pipe are connected, and in which the first acid fed from thefirst stock solution tank and the second acid fed from the second stocksolution tank are mixed and held;

a first valve disposed in the first pipe for adjusting a feed rate ofthe first acid from the first stock solution tank and a second valvedisposed in the second pipe for adjusting a feed rate of the second acidfrom the second pipe;

an acid solution circulation tank to which the third pipe is connectedand that holds the third acid fed from the third stock solution tank;

a mixed acid tank holding a mixed acid solution containing the firstacid and the second acid;

an acid tank holding an acid solution containing the third acid;

at least two fourth pipes linking the mixed acid solution circulationtank and the mixed acid tank for circulating the mixed acid solutionbetween the mixed acid solution circulation tank and the mixed acidtank;

at least two fifth pipes linking the acid solution circulation tank andthe acid tank for circulating the acid solution between the acidsolution circulation tank and the acid tank;

a sheet feeder continuously feeding a steel strip that has been coldrolled and subsequently continuously annealed, and immersing the steelstrip in the mixed acid tank and the acid tank in this order;

a concentration meter measuring iron ion concentration in the mixed acidsolution in the mixed acid tank; and

a controller controlling the first valve and the second valve based onoutput of the concentration meter such as to decrease the feed rate ofthe first acid from the first stock solution tank and increase the feedrate of the second acid from the second stock solution tank, and therebyto lower concentration of the first acid in the mixed acid solution andraise concentration of the second acid in the mixed acid solution as theiron ion concentration in the mixed acid solution rises.

(10) The production system for a cold rolled steel strip according tothe foregoing (9), further comprising

a water tank that holds water and is positioned between the mixed acidtank and the acid tank, wherein

the sheet feeder continuously feeds the steel strip into the water tankafter the steel strip exits the mixed acid tank, and subsequentlycontinuously feeds the steel strip into the acid tank.

(11) The production system for a cold rolled steel strip according tothe foregoing (9) or (10), wherein

the second acid and the third acid are the same type of acid, and thesecond stock solution tank and the third stock solution tank are thesame tank.

Advantageous Effect

The disclosed method of producing a cold rolled steel strip andproduction system for a cold rolled steel strip enable continuousproduction with long-term stability of a steel strip having excellentchemical convertibility, post-coating corrosion resistance in harshcorrosive environments, and surface appearance quality.

BRIEF DESCRIPTION OF THE DRAWINGS

In the accompanying drawings:

FIG. 1 is a schematic view illustrating a cold rolled steel stripproduction system 100 according to a disclosed embodiment;

FIG. 2A is a scanning electron microscope (SEM) image illustrating afilm surface in a comparative example;

FIG. 2B illustrates glow discharge spectroscopy (GDS) analysis resultsfor the comparative example;

FIG. 2C is an image illustrating a sample in the comparative exampleafter testing to evaluate post-coating corrosion resistance;

FIG. 2D is an image illustrating the surface of the sample in thecomparative example;

FIG. 3A is an SEM image illustrating a film surface in Example 1;

FIG. 3B illustrates GDS analysis results for Example 1;

FIG. 3C is an image illustrating a sample in Example 1 after testing toevaluate post-coating corrosion resistance;

FIG. 3D is an image illustrating the surface of the sample in Example 1;

FIG. 4A is an SEM image illustrating a film surface of a sample inExample 2 corresponding to an Fe concentration of 5 g/L;

FIG. 4B is an SEM image illustrating a film surface of a sample inExample 2 corresponding to an Fe concentration of 15 g/L; and

FIG. 4C is an SEM image illustrating a film surface of a sample inExample 2 corresponding to an Fe concentration of 20 g/L.

DETAILED DESCRIPTION Method of Producing Cold Rolled Steel Strip

A method of producing a cold rolled steel strip according to onedisclosed embodiment includes: subjecting a steel strip that has beencold rolled and subsequently continuously annealed to pickling bycontinuously feeding the steel strip into a mixed acid solutioncontaining a first acid that is oxidizing and a second acid that isnon-oxidizing to immerse the steel strip; and subsequently subjectingthe steel strip to repickling by continuously feeding the steel stripinto an acid solution containing a third acid that is non-oxidizing toimmerse the steel strip.

Pickling

In annealing performed to impart desired structure, strength, andworkability on a cold rolled steel strip using a continuous annealingfurnace, a non-oxidizing or reducing gas is normally used as anatmosphere gas, and the dew point is strictly controlled. Consequently,in the case of a normal cold rolled steel strip in which the additiveamount of alloy is small, oxidation of the surface of the steel strip issuppressed. However, in the case of a cold rolled steel strip containing0.5 mass % or more of Si or Mn, oxidation of Si, Mn, and the like, whichare easily oxidized compared to Fe, occurs even if the composition anddew point of the atmosphere gas are strictly controlled duringannealing. Consequently, it is not possible to avoid the formation ofSi-containing oxides such as Si oxide (SiO₂) and Si—Mn-based compositeoxides at the surface of the steel strip. Si-containing oxides areformed not only at the surface of the steel strip, but also at an innerpart of the steel substrate, which impairs etching properties of thesteel strip surface in chemical conversion treatment (zinc phosphatetreatment) carried out as foundation treatment for electrodepositioncoating, and negatively affects formation of a sound chemical conversiontreatment film. Moreover, in recent years, there has been progresstoward the use of a lower chemical conversion treatment liquidtemperature with the aim of reducing the amount of sludge produced inchemical conversion treatment and reducing running cost, and thus it isbecoming the case that chemical conversion treatment is carried outunder conditions in which reactivity of the chemical conversiontreatment liquid with respect to the steel strip is significantlyreduced compared to under conventional conditions. In suchcircumstances, the deterioration of chemical convertibility becomes morenoticeable.

In the pickling of the present embodiment, a cold rolled steel strip iscontinuously fed into a mixed acid solution containing a first acid thatis oxidizing and a second acid that is non-oxidizing to immerse the coldrolled steel strip and remove a Si-containing oxide layer from thesurface of the cold rolled steel strip. The thickness of theSi-containing oxide layer is normally approximately 1 μm from the steelstrip surface, but varies depending on the chemical composition of thesteel strip and the annealing conditions (temperature, time,atmosphere).

The oxidizing first acid may, for example, be nitric acid. The reasonthat the first acid is required in the mixed acid solution is that,among Si-containing oxides, although Si—Mn-based composite oxidesreadily dissolve in acid, SiO₂ displays poor solubility, and thus, inorder to remove this SiO₂, it is necessary to use an oxidizing acid suchas nitric acid so as to remove steel substrate together withSi-containing oxides at the surface of the steel strip.

The concentration of nitric acid in the mixed acid solution ispreferably set within a range of higher than 110 g/L and not higher than188 g/L. This is because a concentration of 110 g/L or lower reduces thepermissible Fe concentration upper limit in the mixed acid solution andshortens the time that continuous pickling treatment can be performedusing the same mixed acid solution without waste liquid treatment,whereas a concentration of higher than 188 g/L makes it difficult todissolve iron-based oxides by the repickling in the subsequent stage.When the concentration of nitric acid is high, the Fe concentration inthe mixed acid solution tends to rise more quickly, and thus thepermissible Fe concentration upper limit tends to be reached morequickly. This shortens the time that continuous pickling treatment canbe performed using the same mixed acid solution without waste liquidtreatment. In view of the above, the concentration of nitric acid ismore preferably 140 g/L or lower, and even more preferably 130 g/L orlower.

The non-oxidizing second acid may be one or more selected from the groupconsisting of hydrochloric acid, sulfuric acid, phosphoric acid,pyrophosphoric acid, formic acid, acetic acid, citric acid, hydrofluoricacid, and oxalic acid. In particular, the use of hydrochloric acid,sulfuric acid, and/or hydrofluoric acid is preferred. The reason forusing a non-oxidizing acid such as described above is to suppress theformation of iron-based oxides that precipitate on the steel stripsurface in accompaniment to pickling with the oxidizing first acid.

The concentration of the second acid in the mixed acid solution ispreferably set within a range of higher than 4.5 g/L and not higher than12.5 g/L. This is because a concentration of 4.5 g/L or lower makes itdifficult to dissolve iron-based oxides by the repickling in thesubsequent stage, whereas a concentration of higher than 12.5 g/Lreduces the pickling weight loss per unit time and may result inresidual SiO₂ in the steel strip surface layer. The concentration of thesecond acid is more preferably 6.5 g/L to 8.5 g/L.

Conditions that influence the amount of Si-containing oxides include thestructure of the steel strip and the annealing conditions. A suitablepickling time for removing Si-containing oxides is determined by takinginto account these conditions. The concentration of nitric acid, thesheet passing speed, and the pickling line length may be set so as toensure this suitable pickling time.

Repickling

Fe that dissolves from the steel strip surface through the picking formsiron-based oxides and these iron-based oxides precipitate on and coverthe steel strip surface, leading to reduced chemical convertibility.These iron-based oxides are removed after the pickling in the presentembodiment by continuously feeding the steel strip into an acid solutioncontaining a third acid that is non-oxidizing to immerse the steelstrip. The term “iron-based oxide” is used to refer to an oxide havingiron as a main component in which the atomic concentration of iron amongconstituent elements of the oxide other than oxygen is 30% or higher.These iron-based oxides are oxides that are present with a non-uniformthickness on the steel strip surface and differ from a natural oxidelayer that is present as a uniform layer of several nanometers inthickness. Note that iron-based oxides formed at the surface of the coldrolled steel strip are known to be amorphous based on observation usinga transmission electron microscope (TEM) and analysis results of adiffraction pattern obtained by electron beam diffraction.

The non-oxidizing third acid may be one or more selected from the groupconsisting of hydrochloric acid, sulfuric acid, phosphoric acid,pyrophosphoric acid, formic acid, acetic acid, citric acid, hydrofluoricacid, and oxalic acid. In particular, the use of hydrochloric acid,sulfuric acid, and/or hydrofluoric acid is preferred. Among these acids,hydrochloric acid is suitable because residual matter tends not toremain at the steel strip surface as with sulfate ions in the case ofsulfuric acid since hydrochloric acid is a volatile acid, and becausethe destructive effect on iron-based oxides by chloride ions is large.Alternatively, an acid obtained by mixing hydrochloric acid and sulfuricacid may be used. The second acid used in the pickling and the thirdacid used in the repickling may be the same type of acid or differenttypes of acids. However, it is preferable to use the same type of acidfrom a viewpoint of simplification of the production system.

The concentration of the third acid in the acid solution is preferablyset within a range of higher than 4.5 g/L and not higher than 12.5 g/L.This is because a concentration of 4.5 g/L or lower makes it difficultto dissolve iron-based oxides, whereas a concentration of higher than12.5 g/L may lead to discoloration due to the presence of residual acidsolution on the steel strip surface. The concentration of the third acidis more preferably 6.5 g/L to 8.5 g/L.

An appropriate pickling time in the repickling is determined based onthe pickling weight loss required to remove iron-based oxides formed bythe pickling in the first stage, pickling efficiency determined by theacid composition, and pickling length. In general, the acid temperatureis approximately 30° C. to 60° C. and the pickling time is approximately10 s.

The cold rolled steel strip that is subjected to the pickling andrepickling described above after continuous annealing may then beprocessed to obtain a cold rolled steel sheet as a product sheet throughnormal processing steps such as temper rolling and a leveling process.

The total pickling weight loss in the pickling and repickling ispreferably 8 g/m² or more. When the total pickling weight loss is 8 g/m²or more, Si-containing oxides and iron-based oxides tend not to remainat the steel strip surface and higher chemical convertibility isobtained.

Control of Acid Concentration in Mixed Acid Solution

The following describes the control of acid concentration in the mixedacid, which is a feature of our techniques. As previously explained,when a cold rolled steel strip is continuously passed along a productionsystem capable of implementing two-stage pickling such as describedabove, and two-stage picking of the cold rolled steel strip is performedcontinuously, as time passes, the surface of the cold rolled steel stripstraight after the pickling in the first stage at that time experiencesa phenomenon of reddish-brown discoloration due to adhered matter.Moreover, we discovered that this phenomenon tends to occur more easilyas the Fe concentration in the mixed acid rises. In other words, wediscovered that as the Fe concentration in the mixed acid rises, thereis an increase in the discoloration area ratio of the surface of thecold rolled steel strip straight after pickling treatment using thismixed acid.

As previously explained, this is caused by a rise in the temperature ofthe mixed acid solution associated with a rise in the Fe concentrationin the mixed acid. Accordingly, in the present embodiment, it isnecessary to appropriately control the pickling rate (i.e., thetemperature of the mixed acid) in accordance with the Fe concentrationin the mixed acid. Specifically, the concentration of the first acid(for example, nitric acid) in the mixed acid solution is lowered and theconcentration of the second acid (for example, hydrochloric acid) in themixed acid solution is raised as the Fe concentration in the mixed acidsolution rises.

In the present embodiment, this control of acid concentration ispreferably used to maintain the temperature of the mixed acid solutionconstantly within a range of 45° C. to 55° C. This is because atemperature of lower than 45° C. reduces the pickling weight loss perunit time and may lead to residual SiO₂ in the surface layer of thesteel strip, whereas a temperature of higher than 55° C. may lead todiscoloration of the steel strip surface starting to occur.

No specific limitations are placed on the configuration by which theconcentration of the first acid in the mixed acid solution is loweredand the concentration of the second acid in the mixed acid solution israised as the Fe concentration in the mixed acid solution rises. Forexample, the following method may be adopted.

The Fe concentration in fresh mixed acid that has not been used forsteel strip pickling is zero. Suppose that the concentrations of thefirst acid and the second acid in the fresh mixed acid are taken to beroughly in the middle of the preferred ranges therefor. For example, theconcentration of the first acid may be set as 132.5 g/L and theconcentration of the second acid may be set as 6.5 g/L.

Thereafter, the Fe concentration in the mixed acid is measured overtime. The Fe concentration may be measured continuously or may bemeasured intermittently at fixed intervals.

The Fe concentration is classified into a number of levels and setconcentrations for the first acid and the second acid are predeterminedfor each level. When the Fe concentration transitions to a next level,the concentrations of the first acid and the second acid are adjusted.For example, at a stage at which the Fe concentration in the mixed acidreaches 15 g/L, the concentration of the first acid may be adjusted to125.0 g/L and the concentration of the second acid may be adjusted to7.5 g/L. As further time passes, at a stage at which the Feconcentration in the mixed acid reaches 20 g/L, the concentration of thefirst acid may be adjusted to 110.0 g/L and the concentration of thesecond acid may be adjusted to 8.5 g/L.

In another configuration, relationship formulae between Fe concentrationand set concentrations for the first acid and the second acid may bepredetermined, and the concentrations of the first acid and the secondacid may be adjusted from moment to moment in accordance with a gradualrise in the Fe concentration in the mixed acid.

Although no specific limitations are placed on the timing of adjustmentof acid concentration in the mixed acid, the values for acidconcentration at each level, and so forth, they may be determined asappropriate in consideration of the composition of the steel strip, theannealing conditions, and so forth.

According to the present embodiment, the control of acid concentrationenables the temperature of the mixed acid to be maintained within apreferred range without an increase in the pickling rate even when theFe concentration in the mixed acid rises. This enables continuousproduction with long-term stability of a cold rolled steel strip havingexcellent chemical convertibility, post-coating corrosion resistance inharsh corrosive environments, and surface appearance quality.

Cold Rolled Steel Strip Production System

The following describes a cold rolled steel strip production system 100according to one disclosed embodiment that can be used to implement themethod of producing a cold rolled steel strip described above. Theproduction system 100 includes, in this order, a water tank 10 thatholds water, a mixed acid tank 12 that holds a mixed acid solution(nitric/hydrochloric acid) containing nitric acid as the first acid andhydrochloric acid as the second acid, a water tank 14 that holds water,an acid tank 16 that holds hydrochloric acid as the third acid, and awater tank 18 that holds water.

A sheet feeder includes rollers 11, 13, 15, 17, and 19 that arerespectively immersed in the five tanks mentioned above and a pluralityof rollers 20 positioned above the tanks. The sheet feeder cancontinuously feed a steel strip P that has been cold rolled andsubsequently continuously annealed and can immerse the steel strip P inthe water tank 10, the mixed acid tank 12, the water tank 14, the acidtank 16, and the water tank 18 in this order.

The production system 100 also includes a nitric acid stock solutiontank 20 that holds nitric acid and serves as a first stock solution tankand a hydrochloric acid stock solution tank 22 that holds hydrochloricacid and serves as a second stock solution tank and a third stocksolution tank. A first pipe 24 extends from the nitric acid stocksolution tank 20, and a second pipe 26 and a third pipe 28 extend fromthe hydrochloric acid stock solution tank 22.

The first pipe 24 and the second pipe 26 are connected to a mixed acidsolution circulation tank 30. In the mixed acid solution circulationtank 30, nitric acid fed from the nitric acid stock solution tank 20 andhydrochloric acid fed from the hydrochloric acid stock solution tank 22are mixed and held. A first valve 32 is provided in the first pipe 24such that the feed rate of nitric acid from the nitric acid stocksolution tank 20 can be adjusted. A second valve 34 is provided in thesecond pipe 26 such that the feed rate of hydrochloric acid from thehydrochloric acid stock solution tank 22 can be adjusted.

The third pipe 28 is connected to an acid solution circulation tank 40.

The acid solution circulation tank 40 holds hydrochloric acid fed fromthe hydrochloric acid stock solution tank 22. A valve is also providedin the third pipe such that the feed rate of hydrochloric acid from thehydrochloric acid stock solution tank 22 can be adjusted.

Two fourth pipes 38 that link the mixed acid solution circulation tank30 and the mixed acid tank 12 are provided as pipes for circulating themixed acid solution between the mixed acid solution circulation tank 30and the mixed acid tank 12. A valve is provided in each of the fourthpipes 38 and these valves enable adjustment of the circulation rate ofthe mixed acid solution. The mixed acid solution circulation tank 30 isprovided with a heat exchanger 36. When the temperature of the mixedacid solution rises due to reaction heat, the temperature can be loweredthrough the heat exchanger 36.

Two fifth pipes 42 that link the acid solution circulation tank 40 andthe acid tank 16 are provided as pipes for circulating hydrochloric acidsolution between the acid solution circulation tank 40 and the acid tank16. A valve is provided in each of the fifth pipes 42 and these valvesenable adjustment of the circulation rate of the hydrochloric acidsolution. The acid solution circulation tank 40 is provided with a heatexchanger 44. A rise in the temperature of the hydrochloric acidsolution due to reaction heat can be suppressed through the heatexchanger 44.

The production system 100 includes an Fe concentration meter 52 thatmeasures the Fe concentration in the mixed acid solution in the mixedacid tank 12. Fe gradually elutes from the cold rolled steel strip overthe course of the pickling, resulting in a gradual rise in the Feconcentration in the mixed acid. The rise in the Fe concentration in themixed acid is detected at appropriate timing by the Fe concentrationmeter 52. For example, the Fe concentration meter 52 may be an analyzerthat, by near infrared spectroscopy, irradiates the mixed acid solutionwith near infrared at intervals of 1 minute and calculates the Feconcentration in the mixed acid solution from the change in the spectrumafter the irradiation. The mixed acid solution fed to the Feconcentration meter 52 may be sampled from the mixed acid tank 12 asillustrated in FIG. 1, or may be sampled from the fourth pipe 38 thatleads from the mixed acid tank 12 to the mixed acid solution circulationtank 30. Note that the production system 100 has a configuration inwhich the mixed acid can be sampled from the circulation tank 30 and fedto the Fe concentration meter 52. This is in order to measure the Feconcentration of fresh mixed acid solution when mixed acid solution inthe circulation tank 30 is replaced.

A controller 54 controls the first valve 32 and the second valve 34based on output of the Fe concentration meter 52. Specifically, thecontroller 54 reduces the feed rate of nitric acid from the nitric acidstock solution tank 20 and increases the feed rate of hydrochloric acidfrom the hydrochloric acid stock solution tank 22 as the Feconcentration in the mixed acid solution rises so as to lower theconcentration of nitric acid in the mixed acid solution and raise theconcentration of hydrochloric acid in the mixed acid solution. Thespecific method of control is as previously described. The controller 54may be implemented by a central processing unit (CPU) in a computer.

Although FIG. 1 illustrates an example in which acid concentration inthe mixed acid is automatically controlled through the controller 54,the disclosed production method is not limited to this example and anoperator may alternatively adjust the first valve 32 and the secondvalve 34 based on measurement results of the Fe concentration meter 52.

A waste liquid pipe 46 extends from the mixed acid solution circulationtank 30 and a waste liquid pipe 48 extends from the acid solutioncirculation tank 40 such as to feed waste liquid to a waste liquid pit50 from each of these tanks. The waste liquid fed to the waste liquidpit is subjected to pH treatment and N₂ treatment in disposal. The Feconcentration in the nitric/hydrochloric acid solution gradually rises,but it is preferable to set the permissible Fe concentration upper limitas a value of 25 g/L or lower. This is because an Fe concentration ofhigher than 25 g/L in the nitric/hydrochloric acid solution makes itdifficult to suppress a decrease in chemical convertibility even throughadoption of our techniques. When the Fe concentration approaches 25 g/L,nitric/hydrochloric acid is discharged to the waste liquid pit 50 fromthe mixed acid solution circulation tank 30, and the mixed acid solutioncirculation tank 30 is replenished with fresh nitric acid andhydrochloric acid from the stock solution tanks 20 and 22. Thepermissible Fe concentration upper limit in the nitric/hydrochloric acidsolution is more preferably set as a value of 15 g/L or lower from aviewpoint of ensuring better chemical convertibility. Moreover, thepermissible Fe concentration lower limit in the nitric/hydrochloric acidsolution is preferably set as 10 g/L or higher from a viewpoint ofoperational efficiency. Although no specific limitations are made aboutdischarge of hydrochloric acid from the acid solution circulation tank40, discharge may be performed at a timing other than during operationonce a certain period of use has elapsed.

As one embodiment, a feed rate A of nitric acid to the mixed acidsolution circulation tank 30 from the nitric acid stock solution tank 20may be set as 0.8 m³/hr to 1.6 m³/hr and a feed rate B of hydrochloricacid to the mixed acid solution circulation tank 30 from thehydrochloric acid stock solution tank 22 may be set as 0.1 m³/hr to 0.3m³/hr. A and B are adjusted at the timing at which the concentrations ofnitric acid and hydrochloric acid are to be adjusted. Moreover, acirculation rate C by the mixed acid solution circulation tank 30 may beset as 25 m³/hr to 90 m³/hr, a waste liquid discharge rate D from themixed acid solution circulation tank 30 may be set as 0 m³/hr to 5m³/hr, a feed rate E of hydrochloric acid to the acid solutioncirculation tank 40 from the hydrochloric acid stock solution tank 22may be set as 1.0 m³/hr to 2.0 m³/hr, a circulation rate F by the acidsolution circulation tank 40 may be set as 25 m³/hr to 90 m³/hr, and awaste liquid discharge rate G from the acid solution circulation tank 40may be set as 0 m³/hr to 5 m³/hr. There is no particular need to adjustC, D, E, F, and G during operation.

Note that by providing the water tank 14 as in the present embodiment,it is possible to prevent nitric/hydrochloric acid carried out from themixed acid tank 12 by the steel strip P becoming mixed into hydrochloricacid in the acid tank 16. This is preferable because it enables reliableremoval of iron-based oxides by repickling in the acid tank 16.

Chemical Composition of Cold Rolled Steel Strip

Although no specific limitations are placed on the chemical compositionof the cold rolled steel strip for which our techniques are adopted, aSi content of 0.5 mass % to 3.0 mass % is appropriate. Si is aneffective element for strengthening steel because it can increase thestrength of steel without significantly reducing workability. However,Si is an element that has a negative impact on chemical convertibilityand post-coating corrosion resistance. To achieve strengthening throughSi addition, it is necessary to add 0.5 mass % or more. Moreover, whenthe Si content is less than 0.5 mass %, the necessity of adopting ourtechniques is low because the impact of poorer chemical conversiontreatment conditions is small. On the other hand, a Si content of morethan 3.0 mass % causes steel hardening, has a negative impact onrollability and sheet passing performance (manufacturability), and leadsto reduced ductility of the steel strip itself. Therefore, Si is addedwithin a range of 0.5 mass % to 3.0 mass %. The preferred range for Siaddition is 0.8 mass % to 2.5 mass %.

No specific limitations are placed on components other than Si and anyvalues within the compositional range of a normal cold rolled steelstrip are permissible. However, in a case in which our techniques areadopted for a high-strength cold rolled steel sheet having a tensilestrength TS of 590 MPa or more that is to be used in an automotive bodyor the like, it is preferable that the chemical composition is asfollows.

C: 0.01 mass % to 0.30 mass %

C is an effective element for strengthening steel and is also aneffective element for forming bainite, martensite, and retainedaustenite having a transformation induced plasticity (TRIP) effect.These effects are obtained through addition of 0.01 mass % or more of C.Moreover, weldability is not significantly reduced so long as theadditive amount of C is 0.30 mass % or less. Accordingly, C ispreferably added within a range of 0.01 mass % to 0.30 mass %. C is morepreferably added within a range of 0.10 mass % to 0.20 mass %.

Mn: 1.0 mass % to 7.5 mass %

Mn is an element that has effects of strengthening steel through solidsolution strengthening, raising quench hardenability, and promotingformation of retained austenite, bainite, and martensite. These effectsare exhibited when 1.0 mass % or more of Mn is added. On the other hand,excessive addition of

Mn leads to increased raw material cost, but addition of 7.5 mass % orless is permissible. Accordingly, Mn is preferably added within a rangeof 1.0 mass % to 7.5 mass %. Mn is more preferably added within a rangeof 2.0 mass % to 5.0 mass %.

P: 0.05 mass % or less

P is an element that has little negative impact on deep drawabilityrelative its significant solid solution strengthening ability and is aneffective element for achieving strengthening. The P content ispreferably 0.005 mass % or more to achieve these effects. On the otherhand, it is preferable to set an upper limit of 0.05 mass % because Pimpairs spot weldability. The P content is more preferably 0.02 mass %or less.

S: 0.01 mass % or less

S is unavoidably mixed into steel as an impurity, and is a harmfulcomponent that precipitates as MnS and reduces stretch flangeability ofa steel sheet. The S content is preferably limited to 0.01 mass % orless and more preferably 0.005 mass % or less in order that stretchflangeability is not reduced. The S content is even more preferably0.003 mass % or less. Industrially, a S content of 0.0001 mass % or moreis obtained in view of desulfurization cost.

Al: 0.06 mass % or less

Al is an element that is added as a deoxidizer in a steel making processand is also an effective element for separating non-metal inclusionsthat reduce stretch flangeability as slag. Therefore, the Al content ispreferably 0.01 mass % or more. However, it is preferable to set anupper limit of 0.06 mass % because excessive Al addition leads toincreased raw material cost. The Al content is more preferably within arange of 0.02 mass % to 0.06 mass %.

In the cold rolled steel strip for which our techniques are adopted, Feand incidental impurities make up the balance exclusive of thecomponents described above. However, the following components mayoptionally be contained.

For example, Ti, Nb, and V are useful elements that not only formprecipitates such as carbides and nitrides and increase the strength ofsteel, but also suppress ferrite growth to refine structure, and improveformability and particularly stretch flangeability. These effects areobtained when 0.005 mass % or more of each of these elements is addedand reach saturation when more than 0.3 mass % is added. Accordingly, itis preferable to add one of Ti, Nb, and V within a range of 0.005 mass %to 0.3 mass %, or to add two or more of Ti, Nb, and V, each within arange of 0.005 mass % to 0.3 mass %. Addition of each of these elementswithin a range of 0.005 mass % to 0.2 mass % is more preferable.

Mo and Cr are elements that improve quench hardenability of steel,promote formation of bainite and martensite, and contribute tostrengthening. These effects are obtained when 0.005 mass % or more ofeach of these elements is added and reach saturation when more than 0.3mass % is added. Accordingly, it is preferable that Mo and Cr are eachadded within a range of 0.005 mass % to 0.3 mass %. Mo and Cr are morepreferably each added within a range of 0.005 mass % to 0.2 mass %.

B is an effective element for raising quench hardenability of steel andcan be added in an amount of 0.001 mass % to 0.006 mass %. Addition of0.002 mass % or less of B is more preferable. Ni and Cu are effectiveelements for strengthening steel and can each be added within a range of0.001 mass % to 2.0 mass %.

N is an element that causes greatest deterioration of an anti-agingproperty of steel and deterioration of the anti-aging property issignificant particularly when the N content is more than 0.008 mass %.Accordingly, the N content should be as small as possible and ispreferably 0.008 mass % or less. The N content is more preferably 0.006mass % or less. Industrially, a N content of 0.001 mass % or more isobtained.

Ca and REM have an effect of causing spheroidization of sulfides and areeffective elements for enhancing stretch flangeability. These effectsare obtained when 0.001 mass % or more is added, but addition of morethan 0.1 mass % reduces cleanliness of steel. Accordingly, it ispreferable that Ca and REM are each added within a range of 0.001 mass %to 0.1 mass %.

EXAMPLES

Operations according to the followings examples and comparative examplewere performed using a production system that was the same as theproduction system illustrated in FIG. 1 with the exception that acontroller was not included. A cold rolled steel strip that had achemical composition containing, in mass %, 0.125% of C, 1.40% of Si,1.90% of Mn, 0.02% of P, and 0.002% of S, the balance being Fe andincidental impurities, and that had been annealed under a reducingatmosphere in a continuous annealing furnace was passed along theproduction system and was subjected to pickling and repickling.

Comparative Example

The concentration of nitric acid in the mixed acid was set as 132.5 g/Land the concentration of hydrochloric acid in the mixed acid was set as6.5 g/L. The Fe concentration in the mixed acid at the start ofoperation was 0 g/L. Although the Fe concentration gradually rose overthe course of operation, the nitric acid concentration and hydrochloricacid concentration in the mixed acid were not adjusted. Theconcentration of hydrochloric acid in repickling was set as 3 g/L. Asample was taken from the steel strip at a section that had been pickledonce the Fe concentration in the mixed acid solution reached 20 g/L andhad subsequently been repickled. The sample was subjected to evaluationas described below. The total pickling weight loss in the pickling andrepickling was 5.9 g/m².

Example 1

At the start of operation, the concentration of nitric acid in the mixedacid was set as 132.5 g/L and the concentration of hydrochloric acid inthe mixed acid was set as 6.5 g/L. The Fe concentration in the mixedacid at the start of operation was 0 g/L. Since the Fe concentrationgradually rose over the course of operation, the concentration of nitricacid was adjusted to 125.0 g/L and the concentration of hydrochloricacid was adjusted to 7.5 g/L at a stage at which the Fe concentration inthe mixed acid reached 15 g/L, and the concentration of nitric acid wasadjusted to 110.0 g/L and the concentration of hydrochloric acid wasadjusted to 8.5 g/L at a stage at which the Fe concentration in themixed acid reached 20 g/L. Adjustment of the nitric acid concentrationand hydrochloric acid concentration in the mixed acid was carried out byan operator. The concentration of hydrochloric acid in repickling wasset as 6 g/L. A sample was taken from the steel strip at a section thathad been pickled once the Fe concentration in the mixed acid solutionreached 20 g/L and had subsequently been repickled. The sample wassubjected to evaluation as described below. The total pickling weightloss in the pickling and repickling was 21.3 g/m².

Chemical Convertibility Evaluation

The samples of the comparative example and Example 1 were subjected tochemical conversion treatment under the following conditions. The grainsize of phosphate film chemical conversion crystals and the film masswere measured. A grain size of 5 μm or less and a film mass of 1.0 g/m³to 3.0 g/m³, which are typical control values, were taken to bepreferred ranges. The film surface was observed at ×1,000 magnificationby an SEM to confirm whether there were locations at which chemicalconversion crystals were not present. In addition, GDS analysis was usedto measure depth direction distributions of O, Si, Mn, and Fe in asample surface layer and confirm whether a Si peak was present at thesurface layer.

Chemical Conversion Treatment Conditions

Each sample was subjected to chemical conversion treatment under thefollowing conditions using a degreasing agent “FC-E2011”, asurface-modifying agent “PL-X”, and a chemical conversion treatmentagent “PALBOND PB-L3065” produced by Nihon Parkerizing Co., Ltd. suchthat the film coating weight was 1.7 g/m² to 3.0 g/m².

Degreasing: Treatment temperature 40° C., treatment time 120 s

Spray degreasing and surface modification: pH 9.5, treatment temperatureroom temperature, treatment time 20 s

Chemical conversion treatment: Chemical conversion treatment liquidtemperature 35° C., treatment time 120 s

The mean grain size was 6 μm in the comparative example and 4 μm inExample 1. The film mass was 0.9 g/m³ in the comparative example and 2.5g/m³ in Example 1. FIG. 2A is an SEM image illustrating the film surfacein the comparative example and FIG. 3A is an SEM image illustrating thefilm surface in Example 1. As illustrated, locations at which chemicalconversion crystals were not present were observed in the comparativeexample, whereas chemical conversion crystals were observed uniformly inExample 1. In the GDS analysis results, a Si peak was detected at thesurface layer in the comparative example as illustrated in FIG. 2B,whereas a Si peak was not detected at the surface layer in Example 1 asillustrated in FIG. 3B. These results demonstrate that the comparativeexample had poor chemical convertibility and Example 1 had excellentchemical convertibility.

Post-Coating Corrosion Resistance Evaluation

The samples of the comparative example and Example 1 were subjected tochemical conversion treatment under the conditions described above andwere further subjected to electrodeposition coating on the surface ofthe chemical conversion treatment film using an electrodepositioncoating material “V-50” produced by Nippon Paint Co. Ltd. such as toobtain a film thickness of 25 μm. A cutter was used to form a cross-cutscar of 45 mm in length in the surface of the resultant test piece. Thetest piece was then subjected to a corrosion test in which 90 cycleswere repeated with each cycle comprising salt spraying (5 mass % NaClaqueous solution: 35° C., relative humidity: 98%) for 2 hours, followedby drying (60° C., relative humidity: 30%) for 2 hours, followed bywetting (50° C., relative humidity: 95%) for 2 hours. After this test,the test piece was washed with water and dried, and then a tape peelingtest was performed on the cut scar section. The maximum total peelingwidth both left and right of the cut scar section was measured.Post-coating corrosion resistance can be evaluated as good when thismaximum total peeling width is 6.0 mm or less.

FIG. 2C is an image illustrating the test piece of the comparativeexample after the tape peeling test and FIG. 3C is an image illustratingthe test piece of Example 1 after the tape peeling test. In thecomparative example, the maximum total peeling width was 7.9 mm andpost-coating corrosion resistance was poor, whereas in Example 1, themaximum total peeling width was 5.6 mm and post-coating corrosionresistance was good.

Surface Appearance Evaluation

FIG. 2D is an image illustrating the surface of the sample in thecomparative example and FIG. 3D is an image illustrating the surface ofthe sample in Example 1. As illustrated, the surface in the comparativeexample was discolored reddish-brown, whereas Example 1 did notexperience such discoloring and had good surface appearance.

Example 2

The Fe concentration in the mixed acid at the start of operation was 5.0g/L. Relationships between Fe concentration and concentrations of nitricacid and hydrochloric acid for ensuring the required pickling weightloss were set in advance by the following relationship formulae (1) and(2). The nitric acid concentration at the start of operation was set as132.5 g/L and the hydrochloric acid concentration at the start ofoperation was set as 5.5 g/L. Since the Fe concentration in the mixedacid gradually rose over the course of operation, the concentration ofnitric acid and the concentration of hydrochloric acid were adjusted inaccordance with formulae (1) and (2) in response.

Nitric acid concentration (g/L)=140−1.5×Fe concentration (g/L)   (1)

Hydrochloric acid concentration (g/L)=4.5+0.2×Fe concentration (g/L)   (2)

The concentration of hydrochloric acid in repickling was set as 8 g/L.Samples were taken from the steel strip at sections that had beenpickled once the Fe concentration in the mixed acid solution reached 5g/L, 15.0 g/L, and 20 g/L, and had subsequently been repickled. Thesesamples were subjected to evaluation as described below. The totalpickling weight loss for the pickling and repickling was 11.0 g/m² forthe sample corresponding to the Fe concentration of 5 g/L, 12.0 g/m² forthe sample corresponding to the Fe concentration of 15 g/L, and 12.0g/m² for the sample corresponding to the Fe concentration of 20 g/L.

The samples were subjected to evaluation of chemical convertibility,post-coating corrosion resistance, and surface appearance by the samemethods as for the comparative example and Example 1.

Chemical Convertibility Evaluation Results

FIG. 4A is an SEM image illustrating the film surface of the samplecorresponding to the Fe concentration of 5 g/L, FIG. 4B is an SEM imageillustrating the film surface of the sample corresponding to the Feconcentration of 15 g/L, and FIG. 4C is an SEM image illustrating thefilm surface of the sample corresponding to the Fe concentration of 20g/L. Chemical conversion crystals were observed uniformly in all theimages. Moreover, a Si peak was not detected at the surface layer in GDSanalysis for any of the samples. This demonstrates that Example 2 alsohad excellent chemical convertibility.

Post-Coating Corrosion Resistance Evaluation Results

The maximum total peeling width was 5.2 mm for the sample correspondingto the Fe concentration of 5 g/L, 4.8 mm for the sample corresponding tothe Fe concentration of 15 g/L, and 5.6 mm for the sample correspondingto the Fe concentration of 20 g/L. Therefore, Example 2 had goodpost-coating corrosion resistance in the same way as Example 1.

Surface Appearance Evaluation Results

The surfaces of the samples corresponding to the Fe concentrations of 5g/L, 15 g/L, and 20 g/L were observed. Reddish-brown discoloring was notobserved at the surface of any of the samples and all the samples hadgood surface appearance. However, slight staining was observed on asection of the surface of the sample corresponding to the Feconcentration of 20 g/L, whereas the samples corresponding to the Feconcentrations of 5 g/L and 15 g/L did not suffer from staining and hadextremely good surface appearance. This demonstrates that it ispreferable to set the upper limit for the Fe concentration as 15 g/L.

INDUSTRIAL APPLICABILITY

The disclosed method of producing a cold rolled steel strip andproduction system for a cold rolled steel strip enable continuousproduction with long-term stability of a steel strip having excellentchemical convertibility, post-coating corrosion resistance in harshcorrosive environments, and surface appearance quality. Therefore, acold rolled steel strip produced by our techniques can be suitably usedas a strengthening component of an automotive body, a component for ahome appliance, a building component, or the like.

REFERENCE SIGNS LIST

100 cold rolled steel strip production system

10, 14, 18 water tank

12 mixed acid tank (for nitric/hydrochloric acid)

16 acid tank (for hydrochloric acid)

11, 13, 15, 17, 19, 20 roller (sheet feeder)

20 nitric acid stock solution tank

22 hydrochloric acid stock solution tank

24 first pipe

26 second pipe

28 third pipe

30 mixed acid solution circulation tank

32 first valve

34 second valve

36 heat exchanger

38 fourth pipe

40 acid solution circulation tank

42 fifth pipe

44 heat exchanger

46, 48 waste liquid pipe

50 waste liquid pit

52 Fe concentration meter

54 controller

1. A method of producing a cold rolled steel strip comprising:subjecting a steel strip that has been cold rolled and subsequentlycontinuously annealed to pickling by continuously feeding the steelstrip into a mixed acid solution containing a first acid that isoxidizing and a second acid that is non-oxidizing to immerse the steelstrip; and subsequently subjecting the steel strip to repickling bycontinuously feeding the steel strip into an acid solution containing athird acid that is non-oxidizing to immerse the steel strip, whereinconcentration of the first acid in the mixed acid solution is loweredand concentration of the second acid in the mixed acid solution israised as iron ion concentration in the mixed acid solution rises. 2.The method of producing a cold rolled steel strip according to claim 1,wherein the first acid is nitric acid.
 3. The method of producing a coldrolled steel strip according to claim 1, wherein at least one of thesecond acid and the third acid is at least one selected from the groupconsisting of hydrochloric acid, sulfuric acid, phosphoric acid,pyrophosphoric acid, formic acid, acetic acid, citric acid, hydrofluoricacid, and oxalic acid.
 4. The method of producing a cold rolled steelstrip according to claim 1, wherein the first acid is nitric acid, andthe second acid and the third acid are hydrochloric acid.
 5. The methodof producing a cold rolled steel strip according to claim 4, whereinconcentration of the nitric acid in the mixed acid solution is setwithin a range of higher than 110 g/L and not higher than 188 g/L, andconcentration of the hydrochloric acid in the mixed acid solution is setwithin a range of higher than 4.5 g/L and not higher than 12.5 g/L. 6.The method of producing a cold rolled steel strip according to claim 1,further comprising immersing the steel strip in water after the picklingand before the repickling.
 7. The method of producing a cold rolledsteel strip according to claim 1, wherein the picking and the repicklinghave a total pickling weight loss of 8 g/m² or more.
 8. The method ofproducing a cold rolled steel strip according to claim 1, wherein thesteel strip contains 0.5 mass % to 3.0 mass % of Si.
 9. A productionsystem for a cold rolled steel strip comprising: a first stock solutiontank holding a stock solution of a first acid that is oxidizing, asecond stock solution tank holding a stock solution of a second acidthat is non-oxidizing, and a third stock solution tank holding a stocksolution of a third acid that is non-oxidizing; a first pipe extendingfrom the first stock solution tank, a second pipe extending from thesecond stock solution tank, and a third pipe extending from the thirdstock solution tank; a mixed acid solution circulation tank to which thefirst pipe and the second pipe are connected, and in which the firstacid fed from the first stock solution tank and the second acid fed fromthe second stock solution tank are mixed and held; a first valvedisposed in the first pipe for adjusting a feed rate of the first acidfrom the first stock solution tank and a second valve disposed in thesecond pipe for adjusting a feed rate of the second acid from the secondpipe; an acid solution circulation tank to which the third pipe isconnected and that holds the third acid fed from the third stocksolution tank; a mixed acid tank holding a mixed acid solutioncontaining the first acid and the second acid; an acid tank holding anacid solution containing the third acid; at least two fourth pipeslinking the mixed acid solution circulation tank and the mixed acid tankfor circulating the mixed acid solution between the mixed acid solutioncirculation tank and the mixed acid tank; at least two fifth pipeslinking the acid solution circulation tank and the acid tank forcirculating the acid solution between the acid solution circulation tankand the acid tank; a sheet feeder continuously feeding a steel stripthat has been cold rolled and subsequently continuously annealed, andimmersing the steel strip in the mixed acid tank and the acid tank inthis order; a concentration meter measuring iron ion concentration inthe mixed acid solution in the mixed acid tank; and a controllercontrolling the first valve and the second valve based on output of theconcentration meter such as to decrease the feed rate of the first acidfrom the first stock solution tank and increase the feed rate of thesecond acid from the second stock solution tank, and thereby to lowerconcentration of the first acid in the mixed acid solution and raiseconcentration of the second acid in the mixed acid solution as the ironion concentration in the mixed acid solution rises.
 10. The productionsystem for a cold rolled steel strip according to claim 9, furthercomprising a water tank that holds water and is positioned between themixed acid tank and the acid tank, wherein the sheet feeder continuouslyfeeds the steel strip into the water tank after the steel strip exitsthe mixed acid tank, and subsequently continuously feeds the steel stripinto the acid tank.
 11. The production system for a cold rolled steelstrip according to claim 9, wherein the second acid and the third acidare the same type of acid, and the second stock solution tank and thethird stock solution tank are the same tank.
 12. The method of producinga cold rolled steel strip according to claim 2, wherein at least one ofthe second acid and the third acid is at least one selected from thegroup consisting of hydrochloric acid, sulfuric acid, phosphoric acid,pyrophosphoric acid, formic acid, acetic acid, citric acid, hydrofluoricacid, and oxalic acid.